1. Field of the Invention
The invention relates to a direct injection spark ignition internal combustion engine and a fuel injection method for the same.
2. Description of the Related Art
Homogenous combustion is performed by forming a homogenous air-fuel mixture in each cylinder and igniting it at the ignition time that comes at the end of each compression stroke. During such homogenous combustion, a tumble flow is created in the cylinder and the created tumble flow is maintained active until the ignition time at the end of the compression stroke. By doing so, the movement of air-fuel mixture can be maintained until ignition, and this air-fuel mixture movement accelerates the combustion speed, whereby the homogenous combustion progresses in a good condition.
For example, Japanese Patent Application Publication JP-A-2005-180247A proposes a direct injection spark ignition internal combustion engine that has an intake flow control valve in each intake port. In this engine, the intake flow control valve guides the intake air to flow along the upper wall of the intake port and enter the cylinder. By doing so, a tumble flow having a high intensity is created in the cylinder, so that the tumble flow can remain active until the time of ignition at the end of the compression stroke.
According to the direct injection spark ignition internal combustion engine, however, the passage of the intake port needs to be narrowed down by the intake control flow valve when guiding the intake air to flow along the upper wall of the intake port and enter the cylinder. According to this structure, when the required intake air amount is relatively small, a tumble flow can be created in the cylinder without causing any particular problems. However, when the required intake air amount is relatively large, if the passage of the intake port is narrowed down by the intake flow control valve as described above, it may cause a shortage of intake air. In this case, a tumble flow having a high intensity can not be created in the cylinder.
However, in direct injection spark ignition internal combustion engines, even if such intake flow control valves are not provided, each tumble flow can be intensified by setting the fuel injection direction appropriately and injecting a fuel having a large thrust force at the end of each intake stroke. However, in this case, a portion of the injected fuel having a large thrust force may reach the surface of the cylinder bore wall and attach there by being absorbed by the engine oil. If the temperature of the surface of the cylinder bore wall is high, the fuel absorbed in the engine oil will readily evaporate, so no particular disadvantages are concerned. However, if the temperature of the surface of the cylinder bore wall is low, the absorbed fuel will remain absorbed in the engine oil, thus diluting the engine oil. Further, if the temperature of the injected fuel is too high, the injected fuel easily evaporates due to the decompression boiling effect, and therefore the tumble flow can not be intensified appropriately. Further, the tumble flow can not be intensified efficiently using the injected fuel if the fuel injection direction is inaccurate.